The present invention is in the general field of metallurgy known as pyrometallurgy, meaning that high temperature is employed in obtaining a metal from its compounds.
In the production of metals from metal oxides, reduction by carbon, known as "carbothermal reduction," is well known. Among the carbothermal reduction methods which are well-known are: the high-temperature reaction of carbon with MgO to produce Mg metal and carbon monoxide; and the high-temperature reaction of carbon with Al.sub.2 O.sub.3 to produce Al metal (in an alloying agent) and carbon monoxide. The back-reaction of CO with molten Al alloy is not a problem, but the back reaction of CO with Mg vapor (which forms MgO) is a problem and is difficult to avoid. Carbothermal reductions are sometimes referred to as "direct reductions."
The term "metallothermic reduction" is widely used in the relevant arts and includes processes whereby a given metal oxide is reduced by another metal. For instance, MgO is reduced at high temperature by a reducing agent such as aluminum, calcium, silicon, or various mixtures of these metals with inert, or less active, metals. In these reactions, where there is no carbon present, the Mg metal is more easily recovered in pure from than when a carbothermal reduction is used which produces equivalent molecular amounts of CO along with the Mg vapor and which gives back-reaction problems as the Mg vapor is cooled and condensed. "Aluminothermic" and "silicothermic" reductions are the most popular of the "metallothermic" reductions.
British Pat. No. 922,300 teaches an indirect reduction of MgO by employing a two-step technique comprising, as the first step, a carbothermic reduction expressed as EQU Al.sub.2 O.sub.3 + 3C (in molten alloying agent) 2Al +3CO and then, as the second step, a metallothermic reduction expressed as EQU 3MgO + 3CaO + 2Al.fwdarw.3Mg + 3CaO.multidot.Al.sub.2 O.sub.3
by separating the two steps, contact between hot Mg and CO is substantially avoided, thus the troublesome back-reaction between the two is substantially avoided.
For purposes of conciseness, the term "chemicothermic reduction" or "chemicothermal reduction" is used in this disclosure to mean a two-stage or two-step reaction involving indirect reduction by employing metallothermic reduction of MgO as one stage or step and carbothermic reduction of Al.sub.2 O.sub.3 as the other stage or step.